Bubbleback for FIFOS

Abstract

A method and apparatus for greatly simplifying the circuitry needed to handle the bubbleback situations in FIFO memories includes an additional row of cells added to the memory array. By adding an extra row of memory cells, the read and write pointer are only on the same row when the FIFO is operating near the empty boundary or in fallthrough mode.

We claim: 
     
       1. An apparatus for preventing the overwrite of information in a FIFO memory having N rows of cells, a write pointer and a read pointer, wherein the distance between said read pointer and said write pointer indicates the fullness of said FIFO, said apparatus comprising: a detection circuit configured to indicate when N-1 rows of cells in the memory are full; and   a disable circuit configured to inhibit the write pointer during times the indication of N-1 rows of cells being full is present.   
     
     
       2. A method for preventing the overwrite of information in a FIFO buffer memory having N rows of cells, a write pointer and a read pointer, wherein the distance between said read pointer and said write pointer indicates the fullness of said FIFO, said method comprising the steps of: indicating when N-1 rows of cells in the memory are full; and   inhibiting said write pointer during times when the indication of N-1 rows of cells being full is present.   
     
     
       3. A method for preventing the overwrite of information in a FIFO memory having N+1 rows of cells and having a write disable circuit operative when N rows of cells contain data, said method including the steps of: reading from the Nth row of cells;   writing data into row N+1; and   disabling a write pointer during times when the indication of N rows of cells of the buffer memory being full is present.   
     
     
       4. The method according to claim 3, wherein the distance between the read pointer and the write pointer indicates the fullness of said FIFO. 
     
     
       5. The method according to claim 3, further comprising the step of: detecting a full memory when said read pointer is within one row of said write pointer.   
     
     
       6. An apparatus for preventing the overwrite of information in a FIFO memory having N rows of cells and having circuitry for indicating when N rows of cells in the buffer memory are full, said apparatus comprising: memory cell means in addition to the N rows of cells; and   means for disabling a write pointer during times when N rows of cells are full.   
     
     
       7. An apparatus for preventing the overwrite of information in a FIFO memory having N rows of cells, a read pointer and a write pointer, wherein the distance between said read pointer and said write pointer indicates the fullness of said FIFO, said apparatus comprising: at least one memory cell in addition to the N rows of cells;   a detector circuit configured to indicate when N rows of cells are full; and   a flag disable circuit for disabling the write pointer when said N rows of cells are full.   
     
     
       8. The apparatus according to claim 7 further comprising: a circuit for writing to said at least one memory cell in addition to the N rows of cells.   
     
     
       9. The apparatus according to claim 7 wherein said at least one memory cell comprises the last row of the memory. 
     
     
       10. The apparatus according to claim 9 further comprising: a circuit for writing to an Nth row of cells.   
     
     
       11. The apparatus according to claim 10 wherein the Nth row is the last row of the memory.